1. Field of the Invention
The invention relates to a new process for preparing unsaturated ethers of the formula 1 from acetals or ketals of the formula 2 according to the following reaction scheme: ##STR3## where R.sub.1 is H or alkyl having from 1 to 8 carbon atoms,
R.sub.2 is H, CH.sub.3, CH.sub.2 H.sub.5 or Cl, PA1 R.sub.3 is alkyl having from 1 to 8 carbon atoms and PA1 R.sub.4 is H, CH.sub.3, C.sub.2 H.sub.5 or C.sub.3 H.sub.7. PA1 R.sub.1 and R.sub.4 may also be linked to one another to form a 5-membered to 7-membered ring.
The unsaturated ethers are important starting compounds for preparing pharmaceutical products and fragrances.
2. Discussion of the Background
The preparation of unsaturated ethers from acetals or ketals is known in the literature. Thus, U.S. Pat. No. 2,667,517 describes a process which uses a hydrocarbon or chlorine-containing hydrocarbon solvent and uses an aromatic or alkylaromatic sulfonic acid catalyst. In this process the solvent is strongly contaminated by high-boiling compounds which form and, therefore, EP-A-0 197 283 proposes in a similar process, a mineral oil as solvent which is burnt after use.
The process of DE-A-40 39 950 is carried out without solvents at from 160.degree. to 200.degree. C. using a catalyst which consists of an acid and an amine. It is well suited only for certain acetals, those of propionaldehyde, butyraldehyde and valeraldehyde. In this process too, the catalyst is contaminated by high boiling compounds which form. Low-boiling acetals such as, for example, the methyl and ethyl acetals of acetaldehyde or acetone cannot, owing to the high reaction temperatures, be reacted at atmospheric pressure by this process.